German Published Patent Application 38 28 477, titled "A Method and Device for Detecting Malfunctions in an Exhaust-Gas Recirculation System of an Internal Combustion Engine", describes a temperature sensor positioned after an exhaust-gas return valve. If certain conditions exist in the internal combustion engine's control system, that is, if an exhaust-gas recirculation took place for a certain period of time, a diagnosis of the system is made through a momentary scanning of the signal from the temperature sensor in an exhaust-gas recirculation duct.
U.S. Pat. No. 3,850,15 titled "Failure Warning Device for an Exhaust-Gas Recirculation System", provides for a warning to be output in case of a faulty exhaust-gas return valve. The error detection is likewise accomplished by a temperature measurement after the exhaust-gas return valve
U.S. Pat. No. 4,060,065 describes a solution to the problem of estimating the quantity of recirculated exhaust gas based upon measured temperature values in the exhaust-gas return pipe. German Published Patent Application No. 32 20 832 describes a system having the same purpose using corresponding means.
However, known systems are not capable of providing optimum results in every situation. Therefore, the object of the present invention is to provide an exhaust-gas recirculation system which has means for diagnosing this system, and is able to reliably and optimally furnish information about its functioning.